One of the constraints that met farmers is the need for mineral and organic amendments to improve crop productivity.

It is well known that synthetic fertilizers can improve short-term yields. But at what cost? The synthetic fertilizers are the source of a non-negligible pollution. Phosphorus is easily leached into streams and groundwater, which eventually can lead to eutrophication of waterways (1). Nitrogen fertilizers, for their part, contain highly volatile compounds such as N2O, a gas witch global warming potential is 298 times higher than that of CO2 (2).

Moreover, conventional farming that makes extensive use of synthetic fertilizers, is also done in monocultures, relies on heavy tillage and allow a virtually unlimited machinery traffic on the fields in addition of using intensively of phytosanitary products (commonly named pesticides). This type of farming can achieve very high short-term yields, but also leads to soil impoverishment, soil compaction and pollution of waterways and atmosphere (3, 4).

Many researchers and innovators have focused on this issue in order to develop new cropping methods, more environmentally friendly but also more sustainable for soil and yield at smaller costs.

Food autonomy, organic agriculture, small-scale and urban agriculture can be attractive alternatives to conventional agriculture which allow consumers to know the origin and quality of its food (4, 5).

Among the proposed solutions, aquaponic systems allows the combination of fish or shellfish farming (aquaculture) and the cultivation of plants in water (hydroponics). It saves water, avoid the use of synthetic fertilizers and add value to fish excrements. The principle of aquaponics is quite simple :

Edible or non-edible fishes are raised in a pool equipped with a pump to aerate and to circulate the water from the pool to the plants, and back into the pool.

Fishes are fed with organic food from the store, or farmed insects (black soldier flies and compost worms are among the most used) and their ammonia rich manure is dissolved in the pool water.

Pool water, enriched with nitrogen, phosphorus and potassium provided by fish manure, is then directed through a pump to the plants growth beds to provide them water and nutrients. Nitrogen availability for plants is achieved by Nitrosomonas bacteria that convert ammonia into nitrites, which are then converted into nitrates by Nitrobacter bacteria. Nitrogen is available to plants when it is in the form of nitrates.

Plants grow in growth beds containing expanded clay bills. These bills provide the mineral elements needed by plants like potassium, are strong (do not crumble), do not affect pH, absorb water while allowing air to circulate and are a good physical support for nitrifying bacteria. Other substrates may also be used, such as river stones (heavier), pozzolana and expanded shale.

Despite the simplicity of the aquaponics system, some difficulties may be encountered. The key is to maintain a proper ratio between the fish population, the amount of fish food, the bacterial population and the amount of cultivated plants for the appropriate nutrients doses that should be filtered by the plants to ensure clean water for the fishes. Exposed nutrients dificiencies in plants indicate that the fish population must be enhanced to bring plants more nutrients while too high concentrations of nitrites and nitrates in pool water indicate that the amount of plants for filtration is too low. It is important to address nutrient deficiencies (which reduce plant productivity) but also the excess of nitrogen (which reduce fruiting and stimulate vegetative growth) for optimal crop productivity (6, 7, 8). The most difficul deficiencies to diagnose, such as iron or micronutrients, must be studied further according to the cultivated plant species to correct the fertilization organically. Moreover, maintaining an optimal pH for the growth of fishes and plants (neutral to slightly alkaline) is important, and must be well monitored and adjusted. It is also very important to choose the fish species to raise that are well adapted to the system temperature (inside or outside, warm or cold regions) and our requirements (edible or not). The most widely farmed fishes are the rainbow trout, tilapia, barramundi, carp, perch, but many others could also be used (7,9).

The result is a closed system, except for the food intake for fishes, the water inflow to compensate the evaporation, and electricity to run the pumps (low cost). Fish manure is recycled by feeding the cultivated plants then fishes receive purified water. When the system is properly adjust, plants can grow up to three times faster than in ground (7,9).

Feasible on a small scale, aquaponic systems can also be adapted to the industrial level. This is the case of the Urban Farmers farm located in Dreispitz (Bâle) in Switzerland, which annually produces five tons of vegetables and one ton of tilapia, on the roof of a warehouse. The biotic and abiotic datas are maintained and monitored by a software developed by the Urban Farmers (10).

USGS (2001) US Geological survey. Selected findings and current perspectives on urban and agricultural water quality by the National water-quality assessment program. Washington (DC): US Department of Interior.